This invention relates to wet process chemistry treatment of substrates, e.g., semiconductor wafers and the like, and is more particularly directed to a technique for plating a flat workpiece in a manner that is efficient and also minimizes surface defects. The invention also concerns a technique that facilitates robotic handling of the articles.
Electroplating plays a significant role in the production of many rather sophisticated technology products, and has recently begun to be used for metallization of semiconductor devices. Recently there has been interest in using plating techniques to form copper conductors on silicon to increase the power or speed of the semiconductor devices.
A number of techniques for electro-depositing or coating on an article face been described in the patent literature.
A recent technique that employs a laminar flow sparger or injection nozzle within the plating bath is described in my recent U.S. Pat. No. 5,597,460, granted Jan. 28, 1997. The means described there achieve an even, laminar flow across the face of the substrate during the plating operation. A backwash technique carries the sludge and particulate impurities away from the article to be plated, and produces a flat plated article of high tolerance, such as a high-density compact disc master or semiconductor wafer. The techniques in that patent improve the flow regime for the plating solution within the tank or cell, as the flow regime is regarded as being crucial for successful operation. Flow regime is affected by such factors as tank design, fluid movement within the process vessel, distribution of fluid within the vessel and at the zone of introduction of the solution into the vessel, and the uniformity of flow of the fluid as it is contacts and flows across the substrate in the plating cell.
In the plating cell as described in said U.S. Pat. No. 5,597,460, a plating bath contains the electrolyte or plating solution, in which the substrate to be plated is submerged in the solution. A sparger or equivalent injection means introduces the solution into the plating bath and forms a laminar flow of the electrolyte or plating solution across the surface of the substrate to be plated. A circulation system draws off the solution from the anode chamber, together with any entrained particles, and to feeds the solution through a microfilter so that all the particles of microscopic size or greater are removed from the plating solution. Then the filtered solution is returned to the sparger and is re-introduced into the plating cell.
The flow regime as described in said U.S. Pat. No. 5,597,460 is further improved by the geometry of the well that forms the tank for the plating bath. The well has a cylindrical wall that is coaxial with the axis of the substrate. This arrangement was intended to avoid corners and dead spaces in the plating cell, where either the rotation of the substrate or the flowing movement of the plating solution might otherwise create turbulences.
An increased evenness in plating is achieved by the technique of my U.S. Pat. No. 5,634,564 in which a rotary blade or wiper is positioned in the plating bath.
Electroless plating is favored in many applications, and especially in those where there is no electrically conductive layer that could serve as a cathode. Accordingly, electroless plating is now seen as an economical alternative to sputtering or vacuum deposition. This is especially true for metals that are difficult to deposit using sputtering or plasma techniques. One advantageous approach to electroless plating is disclosed in my earlier U.S. Pat. No. 5,865,894. In that arrangement, a megasonic transducer adjacent the floor of the plating cell applies megasonic energy at a frequency of about 0.2 to 5 MHz to the solution. The frequency can be above 1 MHz, and in some cases above 5 MHz. The megasonic waves distribute the solution evenly on the substrate, and also break up any bubbles or concentrations that may lead to defects in the plated surface.
Megasonic plating technique can improve the process for electroplating silicon wafers, and an example of this technique, in which the flow regime is further improved by imposing a rotary motion, as described in my U.S. Pat. No. 5,865,894. The megasonic transducer and the rotary blade can be incorporated together in a plating cell, as described and illustrated in my pending U.S. patent application Ser. No. 08/954,239, filed Oct. 20, 1997 now U.S. Pat. No. 5,904,827.
The techniques described in my U.S. Pat. No. 5,932,027 permit mounting the substrate and lowering the substrate into the plating cell to be automated or robotized. Automation and robotization of the insertion, removal, and transport of the workpiece from one process cell to another make it possible to conduct the entire multiplestep plating operation in a clean or super-clean environment. In the technique of that application, the carrier for the substrate is disposed on a sealable door for the plating cell. The door opens to a loading position, which is preferably the horizontal position, and closes to a position which preferably holds the substrate vertically in the plating chamber. The door sealably seats onto an opening in a side wall of the cell. For electroplating use, a cathode ring may be disposed at the periphery of the door opening for making electrical contact with the substrate when the door is closed. This arrangement can lend itself to robotization of the plating process, but nevertheless requires the transfer of the substrate from a transfer holder to a platen associated with the plating cell. Moreover, mechanical and fluid handling considerations must be addressed because of the need to move the substrate between horizontal and vertical orientations.
High precision electroplating in the past has required either rotation of the substrate or rotation of a wiper to induce the removal of hydrogen bubbles from the surface to obtain uniform plating free of defects. For this reason the substrate had to be positioned either in a vertical orientation or in a diagonal or slant orientation, rather than horizontal, so that the bubbles would not collect on the surface. There has not been an effective technique for plating wafers or other substrates in a horizontal, circuit-side-down, orientation.